And I think the 40 dash nowadays is being able to see players' talents on a common event and seeing how they handle the responsibility of prepping for the event.
At this point, every major college team and all the pro scouting departments have the 40 sliced and diced to be able to judge starting reaction, acceleration, and top-end speed with expectations from game tape. They probably can also judge who's been using their time since the bowl game to adapt to ideal running form; versus who gained some chub, is hiding a slight limp every fifth step, and pisses straight THC. Then they all get to either overanalyze or ignore (or both simultaneously) what they saw.
If the combine had all players push a grocery cart up an icy sidewalk in dress shoes, the scouts would find some way to apply that to football skills as well